Fast food giant McDonald's is going on a diet, unveiling a slimmed down version of the meal it markets to children.

But at this stage its healthier Happy Meals, which include fruit and a smaller serve of fries, will only be available in the US.

Nutritionists say that decision will cost Australian children dearly.

After years of fierce lobbying by consumer groups and public health advocates about the nutritional content of meals aimed at children, McDonald's in the US has finally announced it will halve the serving of French fries and add apple slices to its Happy Meals.

"We're excited about it and we think that both parents and children are going to absolutely love the changes that we're making," said Jan Fields, the company's US president.

She told America's ABC News the change will begin in September, with all 14,000 restaurants throughout the US serving the new meals by April.

"I am confident about the changes that we're making on our Happy Meals; it's a 24 per cent reduction in the calories," Ms Fields said.

But the controversial drawcard toy will not be going anywhere.

"Well, the toy is an important part of the experience - come on. Kids have to have fun, you know," Ms Fields said.

Nutritionists in Australia have welcomed the trimming down of Happy Meals.

"I think it's a great thing. I think any move that gets another serve of fruit into our kids, in front of little kids too, is a great move," said Catherine Saxelby, a nutritionist with the website Foodwatch.

"The bigger the portion, the more people eat, and this has been done with movie popcorn, with M&Ms, with macaroni cheese.

"Reducing the serve size is a good first step."

Though Happy Meals in Australia are available with healthier options, restaurants here will not be taking the same step as the US.

"Over the coming weeks and months we're going to take a look at how the US initiative can integrate into our Australian plans," said spokesman Ron Christianson.

"One of the things I think is important to mention is that since 2007 our Happy Meal options have increased exponentially; so while the US announcement is as I said very exciting, we're going to be taking a look at the overall program very closely over the coming weeks and months and determine what would make the most sense for our market here in Australia."

Mr Christianson says over a third of Happy Meals sold in Australia include healthier options.

"I'm quite surprised to hear that that figure is as high as one third; I mean, that's a really great outcome," she said.

"I'd love to see it maybe 60 or 75 per cent of all Happy Meals before I'd say we're doing well."

Suspicious timing

Samantha Graff, director of legal research at the California-based advocacy group Public Health Law and Policy, says she is suspicious about the timing of the US announcement.

"[It is] partly a realisation that parents don't want a toy as a reward for children to choose unhealthy menu options, particularly again in the middle of this obesity crisis that our nation is facing," she said.

"And not to be too cynical, but there has been pressure from multiple points for the fast food industry to reconsider the practices that have been unquestioned and part of our social fabric for as many years as I can count."

Some of that pressure has come from local governments, including San Francisco and nearby Santa Clara in California, which have passed laws that effectively ban free toy giveaways with meals that do not meet nutritional requirements.

In addition, the Centre for Science in Public Interest filed a law suite in San Francisco alleging that advertising practices at McDonald's are unfair and deceptive.

Ms Graff remains concerned about the use of toys to attract young diners to fast-food restaurants.

"You have an industry that is marketing food, luring children with toys when particularly the young children to whom this type of marketing is directed don't even understand the intent of advertising and they are not even able to process what advertising is," she said.

Energy content

The hamburger giant's announcement comes on the same day that researchers released findings of a study into the effectiveness of a New York regulation that requires fast food restaurants to display the energy content of what they are serving.

Similar regulations are planned for New South Wales and Victoria and are being considered by the South Australian Government.

The study found that only about 15 per cent of diners bothered to read the information, although those who did bought food with fewer kilojoules.

Ms Graff says while that number may seem small, it is not an indicator that labelling fast food in this way is a waste of time.

"There's many goals of this type of intervention that include raising consumer awareness but that also include for example inspiring companies to reformulate their products," she said.

"We have seen product reformulation happen since many labelling has taken hold."

McDonald's is also planning to reduce salt in its US menu by 15 per cent by 2015, with further reductions in salt, sugar, saturated fats and energy by 2020.

In Australia, the company says it is already embarking on a process to improve the nutritional quality of its food.